Williams rose to national prominence in his sophomore year. He averaged 19.5 points and 8.3 rebounds per game. During the 2011 season, Williams led the NCAA in True Shooting percentage and Effective Field Goal percentage, helping lead the Arizona Wildcats basketball team to a regular season PAC-10 championship. Williams was named the Pac-10 Player of the Year. Williams also contributed to the Wildcats' run in the NCAA men's basketball tournament. He made a critical block with less than 2 seconds against Memphis. He had a 3-point play against the Texas Longhorns, which advanced the Wildcats to the Sweet 16 of the 2011 NCAA Men's Division I Basketball Tournament. Williams then had a career high 32 points along with 13 rebounds to lead Arizona in an upset victory over the #1-seeded Duke by a 16-point margin to advance into the Elite Eight, where they fell 65–63 to the future NCAA tournament champion, Connecticut. At the end of the season he was projected to be one of the top picks for the upcoming NBA draft. Sophomore year honors:
Williams announced that he would declare for the NBA draft on April 13, 2011. He was selected with the second overall pick by the Timberwolves in the 2011 NBA draft. During the NBA lockout of 2011, he spent the summer at the University of Arizona taking summer classes to finish his degree. Shortly after the draft, Under Armour signed an endorsement deal with Williams. Williams joined several other NBA players for an exhibition game in the Philippines during the summer of 2011. Williams participated in the 2012Rising Stars Challenge, as well as the 2012 NBA All-Star Weekend Slam Dunk Contest. Williams scored a season-high 27 points on February 28, 2012 in a win over the Los Angeles Clippers. He made 9 of 10 shots from the field and 4 of 4 from three-point range. He led the Timberwolves to victory over the New Orleans Hornets with what was then a career-high 28 points on March 17, 2013.
On November 26, 2013, Williams was traded to the Sacramento Kings for forward Luc Mbah a Moute. He made his debut on November 29, 2013, finishing with 12 points, six rebounds and four assists in a 104-96 overtime loss to the Los Angeles Clippers. On December 9, 2013, Williams scored a career-high 31 points in a victory over the Dallas Mavericks. In the Kings' 2014–15 season finale on April 15, Williams scored a season-high 22 points in a 122-109 win over the Los Angeles Lakers.
On July 9, 2015, Williams signed with the New York Knicks. He made his debut for the Knicks in the team's season opener against the Milwaukee Bucks on October 28, scoring a game-high 24 points in a 122–97 win. On January 12, 2016, he recorded his first double-double of the season with 15 points and 10 rebounds in a 120–114 win over the Boston Celtics. The next day, he tied his career-high of 31 points in a loss to the Brooklyn Nets.
On February 9, 2017, Williams signed a 10-day contract with the Cleveland Cavaliers. That night, Williams played 22 minutes in his debut in Cleveland's 118–109 loss to the Oklahoma City Thunder and scored 12 points, going 3-for-3 from the field and 6-for-8 from the free throw line. He went on to sign a second 10-day contract on February 22, and then a rest-of-season contract on March 4. The Cavaliers reached the 2017 NBA Finals, where they were beaten in five games by the Golden State Warriors.
On December 28, 2017, Williams signed with the Tianjin Gold Lions of the Chinese Basketball Association. In 15 games for Tianjin between early January and early February, Williams averaged 20.0 points, 6.6 rebounds, 1.4 assists and 1.7 steals per game.
Los Angeles Lakers (2018)
On March 9, 2018, Williams signed a 10-day contract with the Los Angeles Lakers. After the contract expired, the Lakers decided to part ways with him.
On October 3, 2018, Williams signed with Bayern Munich of the Basketball Bundesliga and the EuroLeague for the 2018–19 season. In the EuroLeague, Williams averaged 13.4 points and 4.2 rebounds over 29 games. In the BBL, Williams won the championship with Bayern after defeating Alba Berlin in the BBL Finals.
